1. Field of the Invention
The present invention relates to the separation of isoolefins from streams containing mixtures of an isoolefin and the corresponding normal olefin. The present invention is especially useful for the separation of isobutene from streams containing n-butenes by oligomerizing the isobutene. More particularly, the present invention results in a higher diisobutene production and less of the higher oligomers or polymers.
2. Prior Art
Isoolefins of four carbon atoms are difficult to separate from the corresponding normal olefin by simple fractionation because of the closeness of their boiling points. In prior art processes as generally practiced commercially, the isoolefin is selectively absorbed by sulfuric acid and the resulting isoolefin-containing sulfuric acid extract is then diluted and heated or treated with steam to separate the isoolefin.
Isobutene and diisobutene are of significant value having diverse applications, for example, isobutene is one of the comonomers for butyl rubber and diisobutene is an intermediate in the preparation of detergents. The isobutene oligomers are useful as polymer gasoline. The n-butenes are required in pure form for homopolymerization and as feeds for the oxidative production of butadiene. One manner of separating these components is to pass the mixture through what is called a cold acid extraction procedure wherein the stream is fed into a bath of concentrated sulfuric acid. Separation is achieved by virtue of the solubility of the isobutene in the sulfuric acid, the n-butenes and other hydrocarbons present passing overhead, for example as shown in U.S. Pat. Nos. 3,546,317 and 3,823,198.
Other processes have used various catalysts for converting the isobutene to diisobutene which is then easily separated from the product stream. For example, a process using a molecular sieve and elevated temperatures is disclosed in U.S. Pat. Nos. 3,531,539; 3,518,323 employs a supported nickel oxide catalyst; and 3,832,418 employs a Group VI or VIII metal deposited on acidic, amorphous silica-alumina in the same manner.
More recently, U.S. Pat. No. 4,215,011 disclosed the use of acid cation exchange resin in a heterogenous combination reaction-distillation system for the selective dimerization of isobutene in the presence of normal butenes. Although some codimer between n-butenes and isobutene are formed, the reaction is highly preferential for the reaction of isobutene with itself and provides a means to separate isobutene from a C.sub.4 stream with little loss of other normal butenes.
Although the present process is suited to treat other isoolefin-normal olefins mixtures, it is of particular significance for the recovery of product streams with sufficiently low levels of isobutene to be processable to obtain useable n-butenes and particularly butene-1 which is the n-butene isomer employed in homopolymerization to produce polybutene or copolymerization with other monomers and as the preferred feed for oxidative dehydrogenation to produce butadiene-1,3.
The present process relates to fixed bed liquid phase systems wherein the isobutene is removed from a feed stream and a oligomer product of the isobutene recovered. In the prior liquid phase systems, the oligomerization is not easily controlled and in addition to diisobutene higher oligomers are produced, some of which form deposits on the resin catalyst reducing its effectiveness. Furthermore, the production of higher oligomers reduces the amount of the desired diisobutene. The diisobutene is preferred, since it is used for alkylations and is also useful as a gasoline blending stock, whereas the other lower oligomers, i.e., trimers and tetramers are not used for the alkylation and are no better as gasoline blending stock than the dimer.
It is a principal feature of the present process that the amount of isobutene in the stream is reduced to levels sufficiently low to allow further separation of a useful butene-1 product. It is another feature of the present process that a diisobutene production is enhanced compared to other possible oligomers.
It is an advantage of the present invention that a method of suppressing the formation of higher oligomers and polymers has been found in the process of separating isobutene from a C.sub.4 hydrocarbon stream by oligomering the isobutene. It is a particular advantage of the present invention that the dimerization reaction of isobutene is enhanced.
It is a feature of the present invention that enhancement and suppression, respectively are obtained by the addition of a small amount of a relatively non-contaminating material to the reaction system.
Another feature of the present process is the substantial energy saving over the cold acid method of isobutene removal.
These and other features and advantages will become clearer from the following.